Comet fixes

This fixes a handful of bugs that when taken together kept things mostly working!

  - Unify lastListenerTime (which was previously always zero) with
    lastListenTime
  - Make lastRenderTime, delta timestamps, and lastListenerTime
    consistent with each other as they are directly compared
  - More accurately track whether any deltas have been sent
  - Pretend initial render happened at creation time to prevent a second
    render call from running on the initial Listen message
  - Rename devMode to alwaysReRenderOnPageLoad
This render mode seems like it's behaving completely differently, and therefore shouldn't be a flag but rather a separate class altogether.

Is this only intended in cases where `render` and `partialUpdate` are mixed?